the letter Oliver writes starts simply with "Holly".. but the letter Shane reads starts with "Dear"
In the Pilot movie Shane confirms to Rita that her birthday is April 30 but this movie takes place a week before her birthday and it's still far too cold to be the middle of April in Denver. There is still snow on the ground and one day Shane says it's only 8 degrees and even in Denver that is nearly impossible for April.
The divorce papers that Joey signed at the lawyer's office seemed to be an official divorce filing, there really isn't anything else the lawyer would need his signature on. However by filing for divorce there is no way his wife wouldn't have found out, once the divorce is filed with the court they will continue to search for her to serve her with the papers. If the first copy was lost/damaged in the mail they would have sent another copy or serve them to her in person. There is no way you can file for divorce without your spouse finding out unless even the police don't know where the other person is.
The "Dead Letter Office" stamp on Oliver's letter to Holly is as washed out as the rest of the address, but the letter would have been stamped after it was recovered. That stamp should be fresh and clear.